1. Field of the Invention
The present invention relates to a radiation image reading apparatus and more particularly to a radiation image reading apparatus adapted to irradiate an excitation light onto a phosphorescent storage plate on which radiation image information is recorded, and to read out the radiation image information by detecting the light volume of the phosphor light emitted from the phospholescent plate caused by the irradiation of the excitation light.
2. Prior Art
Recently, there has been proposed such a radiation image recording and reproduction system adapted to record radiation image information of an object on a phosphorescent storage plate which is called an "Imaging Plate (IP)", generate a phosphor light by scanning the information on the plate with an excitation light, obtain an image signal of the object by reading the image information by means of a photoelectric transfer means, and display the image information on an appropriate display as a visible image or record it on a film as required. According to this prior recording and reproducing apparatus, exposure to radiation such as X-rays may be mitigated by light sensitivity characteristics of the phosphorescent storage material and the phosphorescence plate may be repetitively utilized as the recorded image may be erased. Furthermore, since the obtained signal from the photoelectric transfer means is digitally processed, the image may be easily processed for the display. Thus, various advantages may be anticipated.
An example of an image reading apparatus employed in the prior radiation image recording and reproducing system described above will now be explained by referring to FIG. 1. It is to be understood that an image reading apparatus may be constructed in such a way that an optical deflector such as a polygon mirror, a galvanometer mirror or the like is employed so as to deflect a laser beam and provide raster-scanning with the laser beam on the phosphorescent storage plate of a flat sheet type. Herein, a reading apparatus which employs a rotary drum winding a phosphorescent plate therearound will be described for convenience.
In FIG. 1, numeral 1 designates a phosphorescent storage plate, and numeral 2 designates a high speed rotary drum with the phosphorescent plate 1 wound therearound. Numeral 3 designates a laser beam source adapted to generate a single laser beam, and numeral 4 a mirror adapted to direct the laser beam from the laser beam source 3 onto the phosphorescent plate 1. Numeral 5 designates a focusing lens, numeral 6 a filter the filtering characteristics of which are set such that the laser beam from the source 3 is shielded to allow only the phosphor light to pass therethrough, numeral 7 a photo multiplier (PMT), numeral 8 a current/voltage (I/V) converter, numeral 9 an amplifier, numeral 10 an analog/digital (A/D) converter, and numeral 11 a display.
In the reading apparatus according to a prior art described above, when the rotary drum 2 is rotated at a high speed, and the optical system comprising the mirror 4, focusing lens 5, filter 6 and photomultiplier 7 is shifted in parallel in the axial direction of the rotary axis of the rotary drum 2 (in the direction indicated by an arrow "A" as shown in FIG. 1), the laser beam may be raster-scanned on the phosphorescent plate 1. The phosphorescent plate 1 is caused to emit light by irradiation with the laser beam, and the emitted light is focused by the focusing lens 5 and detected and converted into a current signal at the photomultiplier 7. The obtained current signal of the radiation image information will be converted to a voltage signal at the I/V converter 8 and input to the A/D converter 10 via the amplifier 9 whereby it is converted to a digital signal, and input to the image display 11. In this way, the image read from the phosphorescent plate 1 can be displayed in the display apparatus 11 as a visible image and stored in an appropriate information recording medium, for example a film, as required.
It is to be understood that the intensity distribution of a single laser beam is supposed to take a form generally similar to Gaussian distribution as shown in FIG. 2A and form a wide base part. Besides, since the above-mentioned reading apparatus is based on a destructive scanning method, the image which has been stored on the phosphorescent plate 1 will likely be lost if it is irradiated by the excitation light so as to read out the image. The more intense the excitation light is, the greater the amount of the stored image, which is lost.
Accordingly when the laser beam is scanned on such a slit image as shown in FIG. 2B, the base part of the laser beam may have reached the slit image before the peak of the laser beam arrives at the slit image, and the phosphorescent plate 1 may have been excited thereby. As a result, a certain amount of phosphor light has to be emitted from the slit part of the plate before the image should be exactly scanned. In other words, light emission may be caused before the peak of the beam arrives at the front part of the slit image, and the image at the front part is destroyed to a certain extent so that the reproduced image becomes obscure at the front part of the slit image when the part is exactly read out. On the other hand, since the slit image has been largely destroyed and diminished after the peak of the laser beam has passed through the slit, almost no excitation will be generated by the opposite base part of the laser beam. This will cause the line spread function (LSF) to be asymmetrical as shown in FIG. 2C, or the sharpness in the scanning direction will be degraded more than in the opposite direction not only in the main scanning direction but also in the sub-scanning direction.
It is conceivable to make the configuration of a laser beam having a sharp rise by increasing the focus of the light in order to narrow the base part or decrease the light in the base part. In that case, however, since the excitation light will be irradiated only onto a part of each pixel area in the case of a laser beam source emitting a single laser beam, the entire area of the pixel cannot be excited. Consequently, only a part of the image information stored in the respective pixels may be read out and the reproduced image cannot represent exactly the recorded image.